I don't understand why this is allowed either.

I first noticed when our FC tag word started showing up on non-members' nameplates. Most people probably don't know their choice of tag is already taken, but it's becoming more common, even for far less common words than "Kupo". There might not be much SE can do for similar tags ("Kupo!" is different than "Kupo?" or "Kupo"), but identical tags and names (even if different in capitalization) really need to be prohibited.